This invention relates to high efficacy, low wattage metal halide arc discharge lamps, and more particularly to an apparatus including a low wattage bulb and associated operating circuitry therefor. This application is directed to a DC ballast metal halide arc discharge lamp apparatus.
In the prior art metal halide lamps have generally been of the so called higher power type, usually over 35 or 40 watts and have included rather involved and complicated, as well as expensive control circuitry and apparatus for operating metal halide arc discharge lamps. Discharge lamp inductive/capacitive ballasts have tended to be, heavy per kilowatt of total power, have high power consumption (10%-20% of total power input) and expensive. As the demand for low wattage metal halide lamps having high efficacy, i.e., high lumens per watt, has increased, it has become increasingly important to create new and improved circuits and apparatus for starting and operating metal halide lamps in a more efficient fashion, without sacrificing efficacy. It also has been necessary in miniature applications of these low power lamps, where they are frequently installed in inaccessible locations, to provide means for automatically compensating for aging of the bulb so that the rated light output can be maintained for the life expectancy of the bulb. Further, as the size of the lamp has decreased, many portable applications have become desirable in which the physical orientation of the metal halide bulb will frequently change. It is known that changes in physical positioning of a metal halide bulb will change the operating characteristics and hence light output.
Metal halide arc discharge lamps have three basic control requirements, namely, sufficient starting voltage, lamp current regulation and for AC operation relighting of the lamp each half cycle. Because of these requirements, ballasts have tended to have large transformers, inductors and capacitors connected between the discharge lamp and a power supply, with feedback circuits, special variable magnetic paths in the transformers, and multiple transformer windings.
Also, since the metal halide low wattage lamp is particularly adapted for miniature applications involving access to remote locations, frequently involving dangerous or explosive atmospheres, safety of operation has become a concern. Traditional prior art control systems for discharge lamps generally were not concerned with the safety aspects of discharge lamp operation.